Optical elements capable of reflecting a beam of x-rays in preselected directions are useful in a variety of apparatus including x-ray monochromators, x-ray analyzers, x-ray imaging systems such as x-ray microscopes and the like. X-ray fluorescence is widely used for the qualitative and quantitative analysis of a variety of materials and the technique relies upon the use of an x-ray reflective element to resolve a multiple wavelength flux of x-rays into its component wavelengths through Bragg reflection. Bragg reflection is a phenomenon well known in the art and occurs when a beam of energy, such as x-rays is reflected from a series of planes of a periodic structure, such as a crystal. The reflections from the multiple planes establish an interference condition and the reflected wavelength will depend upon both the angle of incidence, .theta. and the spacing of the periodic structure referred to as "d". The reflected wavelength will be defined by the Bragg equation: EQU n.lambda.=2d sin .theta.
wherein n is the order of the diffraction. It will thus be appreciated that such x-ray reflective structures can be the functional equivalents of elements such as diffraction gratings, prisms, mirrors or lenses which are used at visible wavelengths.
Natural crystals have previously been employed as x-ray reflective elements; however, the utility of these materials has been limited by the fact that the d-spacing of the crystalline planes is defined by the lattice parameters of the crystal. Also, a number of crystalline materials are unsuitable because they do not adequately reflect the appropriate wavelengths of x-rays and/or because they fluoresce or otherwise interfere with the intended use of the reflective element.
Natural crystals generally have lattice spacings which do not significantly exceed 10 angstroms. These spacings are adequate for fairly high energy x-rays; however, in many instances it is desirable to measure the x-ray fluorescence of relatively light elements; therefore, relatively soft, long wavelength x-rays must be employed thereby necessitating d-spacings significantly larger than those found in natural crystals. Toward that end, the art has investigated a number of synthetic structures. The earliest structures were comprised of molecular layers of heavy metal soaps, such as lead myristate or lead stearate. These materials are referred to as Langmurr-Blodgett (LB) films. While they can provide large lattice spacings, their lattice parameters are limited to specific values. Furthermore, the materials are soft, and difficult to prepare and are very unstable in ambient conditions; they also tend to decompose under high fluxes of x-rays.
Another approach involves the use of multilayered thin films. These reflective structures comprise a plurality of stacked layer pairs. One member of each pair comprises a material having a very high x-ray reflectivity, and the second member of the pair, often referred to as a spacer, comprises a material having a lower reflectivity. In this manner, there is provided a periodic structure which is the one dimensional analog of a crystal. It will be appreciated that the thicknesses of the layers may be controlled so as to provide for a great deal of selectivity in the spacing. Such structures, and techniques for their manufacture, are disclosed in U.S. Pat. Nos. 4,693,933; 4,727,000 and 4,785,470, the disclosures of which are incorporated herein by reference.
Synthetic multi-layer structures are widely used in x-ray fluorescence analyzers; however, the performance of presently available multi-layer x-ray optical elements is less than adequate with regard to the analysis of relatively light elements such as nitrogen, oxygen and fluorine. This is because of the fact that the K shell fluorescent emission from these elements constitutes relatively soft x-radiation, and an element optimized for the Bragg reflection of these wavelengths requires a spacer material which has a relatively low absorption for soft x-rays. Also, operation at soft wavelengths, particularly those associated with the "water window" is important for x-ray microscopy; and consequently there is a need for imaging optics operative in this range. Multilayer technology allows for the coating of spherical, cylindrical, or other curved and irregular shapes so as to provide unique optical elements. Also, synthetic, multilayered structures may be fabricated with a graded d spacing wherein d varies across the surface of the structure. Such graded d spacing directs the reflected beam and controls reflection and are particularly useful as focusing elements.
Operation at soft x-ray energies is difficult because many of the conventionally employed spacer materials such as carbon, silicon, magnesium and the like are too absorbing to be part of an efficient reflective element operative at relatively low energies.
In accord with one prior art approach, a multilayer structure of iron and scandium may be employed as a reflective element for relatively soft x-rays. In this structure, iron acts as the reflective layer and scandium as the spacer. Scandium itself is a fairly high electron density material which is generally quite reflective of x-rays; however, scandium has the unusual property of having a resonance in its optical constants which produces a low absorption window at approximately 0.39 KeV. While the iron scandium structure is quite efficient at this specific energy range it is a very poor reflective element at energies only slightly higher and slightly lower. Hence its utility is limited primarily to nitrogen detection in the energy range of 0.39 KeV. In addition to being of relatively limited utility, this structure is somewhat difficult to fabricate. It is difficult to obtain smooth interfaces between the iron and scandium layers, and the poor interface quality degrades device performance. The device also presents problems of stability because of the high reactivity of scandium, especially when in the form of thin layers.
Another prior art approach to this problem is disclosed by Boher et al. in a paper entitled "Tungsten/Boron Nitride Multilayers for X-UV Optical Applications" published in SPIE, vol. 1546, Multilayer and Grazing Incidence X-ray/EUV Optics (1991) pp 520-536. Disclosed in this reference is the fabrication of tungsten/boron nitride x-ray reflective elements by reactive radio frequency diode sputtering. As noted therein, the boron nitride tends to decompose during the R.F. diode sputtering process; and consequently, a specifically controlled amount of nitrogen must be added to the deposition environment to control the stoichiometry of the resultant layer. Because of the difficulty in the control of the process and the lower than desired reflectivity of the resultant structure, there is still a need for an easy to fabricate an efficient, x-ray reflective element which can be used in the x-ray fluorescence analysis of relatively light elements; and most particularly in the wavelength region of 23-43 .ANG. (normal incidence), as is typically encountered in the case of x-ray imaging optics such as in an x-ray microscope.
As will be described in further detail herein below, the present invention provides for improved x-ray optical elements having boron nitride based spacer layers, fabricated by a process which is easy to implement and control. The optical elements are highly efficient, environmentally stable and usable over a relatively wide energy range. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow: